1. Field of the Invention
The present invention relates to a surface protective film of a color filter for light-receiving elements and display elements.
2. Description of the Related Art
As surface protective films for color filters for light-receiving elements and display elements, surface protective films of, for example, acrylic polymer (Electronic Material, pp. 53 to 60, Dec., 1988), polyimide (Japanese Patent Laid-Open No. Sho 61-254905), and organosilicon ladder polymer having a cage or hydroxyl group type end (Japanese Patent Laid-Open Nos. Sho 61-3120 and Sho 61-3124) are conventionally known.
In manufacturing a color filter for a display element, after a surface protective film of the color filter is formed, a transparent electrode is formed on the surface protective film at a temperature of not lower than 250.degree. C., thereby producing a display element. In manufacturing a color filter for a light-receiving element the light-receiving element is sealed by a resin at 250.degree. C. in the final process of production. When either these elements are mounted on a substrate, they are soldered at a temperature of not lower than 250.degree. C.
In order to protect the color filter, it is therefore necessary that the surface protective film formed on the color filter not only has a high light transmittance but also a heat resistance high enough to withstand the above-described heat treatment.
Among the above-described materials for surface protective films are the acrylic polymers which while they have a high light transmittance also have a poor heat resistance, so that the light transmittance sometimes deteriorates by the above-described heat treatment.
Polyimides have an excellent heat resistance but since the inorganic ingredient, which is generated during thermal cure, adheres to the wafer surface and obstructs the subsequent steps, an oxygen plasma treatment is required in order to prevent this phenomenon. The process for producing a protective film thus becomes complicated, and the film formed is inferior in light transmittance. In addition, since the varnish used is a polar solvent, it has a poor moisture resistance making it necessary to keep the varnish in a cool and dark place.
Among the organosilicon ladder polymers disclosed in Japanese Patent Laid-Open Nos. Sho 61-3120 and Sho 61-3124, organosilicon ladder polymers having a cage type end are inferior in adhesiveness with a substrate, so that they sometimes become separated from the color filter. In addition, since they are not cured by heating, they sometimes dissolve in the solvent which is used in the resist removing process after pattern formation. That is, they have a poor solvent resistance. Organosilicon ladder polymers having a hydroxyl group at the end are cured when heated at 150.degree. to 350.degree. C. while DBU (1,8-diazabicyclo-[5,4,0]-7-undecene) is used as a curing catalyst, but the curing catalyst deteriorates the light transmittance of the surface protective film and has a harmful effect on the characteristics of the color filter. In addition, since the organosilicon ladder polymers having a hydroxyl group at the end have a low molecular weight (5.times.10.sup.4) , they have inferior film-forming properties, so that when the film is subjected to various treatments after film formation, a crack is apt to be produced in the protective film making it impossible to form a thick film.